Modulation techniques find wide application in communications systems. In fact, modulation techniques are used to facilitate the transmission of communications signals over many mediums including wired and wireless systems. For example, modulation techniques are used to transmit communications signals over wired systems such as coaxial or twisted pair. Also, modulation techniques are used to transmit communications signals over the air such as in microwave or satellite communications. In practical applications, it may also be the case that several modulations are necessary to achieve a final result. For example, several modulations may be necessary to transmit a communications signal from customer premise equipment (CPE) to a local office and then to transmit it via microwave systems over long distances.
In achieving microwave communications it is often necessary to modulate a signal for transmission over a cable and then re-modulate it for transmission over the air. Toward optimizing modulation for cable transmission a relatively low modulation frequency is typically used. But, to optimize modulation for over-the-air transmission, relatively high microwave frequencies are preferred. Thus, the relatively low frequency modulation must be re-modulated to a high microwave frequency. Microwave transmitters, however, typically have narrow range tuners such that several re-modulations may be necessary.
Prior attempts have been made to solve the problem of translating a modulated signal from a relatively low modulation frequency to a relatively high modulation frequency by using a heterodyne technique. In this technique, a first modulation at a relatively low frequency is implemented for transmitting a communications signal over a cable. At the other end of the cable, the signal at the relatively low modulation frequency is then re-modulated to a relatively high frequency. Here, the first modulation frequency needs to be low enough in frequency so that the interconnecting cable loss is minimized. Among other things, this heterodyne approach requires carefully designed image rejection filters to achieve adequate frequency translation. For good image rejection, the first modulation frequency must be as high as possible to make the image rejection filter easier to build which is in conflict with the need to keep the first modulation frequency as low as possible. The presence of the image rejection filter greatly reduces the ability to tune the second modulator to an arbitrary frequency. This technique, developed many years ago, is still quite common in many radio products from a variety of manufacturers.
A second technique, however, places the modulation source at the end of the cable near the microwave transmitter and uses direct conversion to modulate to a relatively high frequency suitable for application to a microwave transmission. Direct conversion by its nature allows for a wide tuning variation. Data, however, must be transmitted through the cable so that it can be modulated at the other end of the cable. To do this requires additional hardware to detect the data and correct for signal transmission impairments induced by the cable as well as to recover data and the timing information associated with it. This direct modulation approach suffers from the fact that data and clock need to be applied to a modulator at the cable end. Therefore, a modulator is required to drive the cable and a demodulator is required at the cable end to receive the data and regenerate the timing information associated with that data.
Accordingly, there is a need to improve modulated signal communication methods and systems. The present invention addresses the foregoing and related issues.